I have a "crazy' theory. So you'll want other people's stories as well.
The story I see looks something like this.
Housing bubble and runup in housing construction from 2000-2006. Housing price bubble pops in 2007. From mid-2007 to mid-2008 we have a garden-variety recession.
From June to August 2008, a series of shocks hit the American economy.
(1) Continued fallout from the housing bubble which ripples through the financial sector. Nobody has any clear plan of what to do with very large problem banks. Consider this a "demand" shock to money velocity.
(2) The oil price spike in June 2008 was "exogenous" to the American economy and represented a mild supply shock.
(3) Contemporaneously, "confidence" falters as private expectations of future income fall dramatically. (source: U Michigan Survey of Consumers). A 'demand shock" to consumption.
(4) real interest rates soar upward (source: look at the yield on inflation-indexed bonds as a proxy for real interest rates). A "demand shock" to investment.
As a result of these shocks the demand for money (or, equivalently, safe assets like Treasury bills) increased.
However the Fed and Treasury did not act to meet that increased demand. Monetary policy loosened in absolute terms but not relative to where they needed to be.
The "crash" in October-November 2008 was precipitated by a contraction in aggregate demand caused by tight money, falling expectations of future income and soaring real interest rates.
Monetary policy continues to be tight relative to where it needs to be, even today. That's why unemployment is so high and output so low. We can also see this in tepid realized inflation, low inflation expectations and continued low asset prices.
In short: mild "real" recession from mid-2007 to mid-2008. Tight monetary policy (relative to where it needed to be) drove the economy off a cliff in June-August 2008. Large recession followed.
But again, this is a story that I piece together using evidence from the Great Depression and the 1987 stock market crash. The basic causality is "tight money -> financial crash -> recession." It's not the financial crash that caused the recession, it's tight money.
What I need to do now is piece together a causal story that is plausible to people other than monetary economists.